Shadowy Tormentors inhabit this dimension, and emerge from it to torment death knights such as Koltira Deathweaver. The Lich King originally kept them at bay, but after the Knights of the Ebon Blade left his service, the tormentors are now loose upon them. He even appears in the World of the Dead, as he calls it, riding a frost wyrm during  The Power to Destroy.
Alpha Beta PTR Vanilla 1.1.0 1.2.0 1.3.0 1.4.0 1.5.0 1.6.0 1.6.1 1.7.0 1.8.0 1.8.4 1.9.0 1.10.0 1.11.0 1.12.0 Burning Crusade 2.0.1 2.0.3 2.0.4 2.1.0 2.1.2 2.2.0 2.3.0 2.4.0 2.4.3 Wrath of the Lich King 3.0.2 3.0.3 3.0.8 3.1.0 3.2.0 3.2.2 3.3.0 3.3.3 Cataclysm 4.0.1 4.0.3a 4.0.6 4.1.0 4.2.0 4.2.2 4.3.0 4.3.2 Mists of Pandaria 5.0.4 5.0.5 5.1.0 5.2.0 5.3.0 5.4.0 5.4.7 Warlords of Draenor 6.0.2 6.0.3 6.1.0 6.2.0 Legion 7.0.3 7.1.0 7.1.5 7.2.0 7.2.5 7.3.0 7.3.5 Battle for Azeroth 8.0.1 8.1.0 8.1.5 8.2.0 8.3.0 Shadowlands 9.0.1
The spell called "spiritual projection" or "astral projection" brings the caster to the Twisting Nether instead of the Shadowlands; and the Twisting Nether is also referred as "realm of ghosts". However, both planes don't appear to be related, as Outland is floating on the Twisting Nether, but the Shadowlands can be reached in Outland in The Burning Crusade.
In classic World of Warcraft expansion "Wrath of the Lich King," Alliance soldier Bolvar Fordragon replaced Arthas Menethil as the avatar of the Lich King, ruler of the undead Scourge on Azeroth. Pledging to keep the ravenous dead at bay, Bolvar was sealed into the Frozen Throne, entombed in the haunted armor of the Lich King. It looks as though Bolvar is set to play a central role in the next WoW expansion, tentatively dubbed "Shadowlands."
The four zones that comprise the Shadowlands are ruled by "Covenants", similar to the Class Orders introduced in Legion. Each Covenant will have its own campaign, similar to the War Campaign in Battle for Azeroth, with gear specific to the faction and abilities both universal and determined by class. The four covenants are the Kyrian of Bastion, the Night Fae of Ardenweald, the Venthyr of Revendreth, and the Necrolords of Maldraxxus. Players experiencing the Shadowlands content for the first time will need to reach level 60 before they can choose to pledge themselves to a Covenant, while alts are able to do so at the start of the Shadowlands content.
Adventurers entering the Shadowlands will find the realm of the dead in disarray. In the natural order of things, souls are sorted and sent on to an afterlife realm appropriate to the lives they lived, but starting around the time of the Burning Legion's third invasion of Azeroth, all souls who have perished—including the innocents slain at Teldrassil—are being funneled directly into the Maw. The Shadowlands are starving for anima even as the Maw continues to grow from the glut of fresh souls. Sylvanas Windrunner has been seemingly perpetrating acts to bring about great amounts of death and destruction. In partnership with the Jailer, they have been working toward a common end for some time.